We have made two trips to the Cornell High Energy Synchrotron within the last year and a half. Our goals for these trips were threefold : (1) To establish the limit of diffraction of the IgE receptor (FceRIa) crystals (2) To collect a native data set to the highest resolution possible and (3) to collect potential heavy atom derivative data sets for isomorphous replacement. We have made substantial progress for each of these goals. The FceRIa crystals belong to spacegroup P41 or P43 (cell dimensions 145 x 145 x 63_) and diffract X-rays weakly. The c-axis of the crystals can vary from 62_ to 72_, necessitating the collection of multiple native and derivative data sets. We have collected three native data sets at CHESS, two to a resolution of 4.0 -4.5 _, and one to a resolution limit of 3.7 _. For comparison, the diffraction limit using a laboratory X-ray source is lower than 6 _. We expect 2-3 receptor molecules per asymmetric unit and therefore non-crystallographic symmetry averaging should improve the electron density maps at 3.7 _. We continue to improve the FceRIa crystals and data collection strategies in order to extend the data to higher resolution. Finally, we have collected two potential heavy atom derivative data sets for these crystals. Both compounds have shown significant intensity differences in initial screening carried out at the NSLS X-25 beamline, but these crystals also suffered from significant non-isomorphism along the c-axis. We have collected full new data sets at CHESS, which have cell dimensions within 1 - 1.5% of the 3.7_ native data. Initial results from one of these potential derivatives gives mean fractional isomorphous differences of 16% compared to the native and the interpretation of difference patterson maps is underway.